The Real-Ear-To-Coupler Difference (RECD) is a unique acoustical measure for an individual ear that allows for prediction of sound pressure level (SPL) at the eardrum by using the results of SPL measured in a 2-cc coupler. Briefly, in order to quantify measurements related to the ear, it is common practice to simulate an average ear using certain mechanical and acoustical systems. An ear simulator (like a coupler) is an example of such a simulation system, having the same acoustic input impedance as an average occluded ear.
The main purpose of RECD is to simplify the procedure of evaluation of the ear canal SPL of an individual. The measure of difference in SPL directly reflects the difference in impedances between the individual ear and the 2-cc coupler. If the sound pressure of the occluded ear (e.g. an ear having a hearing aid inserted therein) was the same at the reference plane (R.P.) of the ear (e.g., at the ear tip) and at the eardrum, then a ratio of impedances of the occluded ear and the coupler could be used to define RECD. But it is well known that the sound pressure along the occluded ear canal will change rapidly at high frequencies because of forming standing waves. Therefore, one must distinguish between the input and the output of the occluded ear (i.e., the reference plane and the ear drum, respectively), which requires the use of the transfer impedance.
A description of obtaining an RECD with a high impedance sound source and an HA-1 coupler can be found in a review chapter of a book by Lawrence J. Revit, edited by Michael Valente, entitled “Strategies for Selecting and Verifying Hearing Aid Fittings”, 2002, Chapter 3: Real-Ear Measures, second edition, Thieme Medical Publishers, Inc., New York and Stuttgart, 2002, which is incorporated by reference herein. The measurement procedure includes:                1. Attach the high-impedance sound source (such as, an insert earphone or a hearing aid) with the ear tip (usually surrounded with foam) to an HA-1 coupler. Measure and record the sound pressure registered by a probe microphone (2-cc SPL probe).        2. Detach the foam ear tip from the HA-1 coupler and insert it into the ear canal. Measure and record the SPL registered by the probe microphone (Ear canal SPL Probe).        3. Calculate RECD as eardrum response as RECD=(Ear canal SPL) minus (HA-1 SPL).        
RECD measurement errors due will occur if the probe microphone is placed at a distance from the eardrum. The occluded ear canal can be presented as a tube with a length L that is individual for each person. Such a tube in combination with elements that simulate the impedance of the middle ear (eardrum, incus, cochlea, etc.) creates ear canal resonance and associated standing waves. Due to the standing waves, the sound pressure at the eardrum will be different from the sound pressure at a distance from the eardrum. FIG. 1 is a simplified mode of an ear canal and the sound pressure distribution in case of a ¼ wavelength standing wave.
The standing waves in the occluded ear canal will cause RECD measurement errors that depend on the depth of insertion of the probe microphone (further described in a technical application note by Per V. Bruel, et al., “Impedance of Real and Artificial Ears”, 1976, Bruel & Kjaer Sound & Vibration Measurement A/S, Denmark). The shorter the distance from the eardrum to the probe microphone, the fewer amount of errors will occur. Simulated RECD errors for different positions of the probe microphone in the ear canal (11 mm for the length of the ear canal×7.5 mm for the diameter of the ear canal) are shown on FIG. 2. The occluded canal length is 11 mm (an average ear canal of an adult). The sound pressure SPL is normalized over the sound pressure SPL at the eardrum.
The Revit reference, cited above, recommends placing the probe microphone not farther than 6 mm from the eardrum so that the RECD errors due to the standing waves will not exceed 2 dB at 6 kHz (sound source frequency) and 4 dB at 8 kHz (sound source frequency). In practice, it is quite difficult to position the probe microphone deep into the ear canal so significant errors could occur if the probe microphone is placed incorrectly.
A recent U.S. Patent Publication (U.S. Patent Publication No. 2010/0202642 by Janice LoPresti and Tao Zhang, entitled “Method To Estimate The Sound Pressure Level At Eardrum Using Measurements Away From The Eardrum”, filed Jan. 11, 2010) describes an alternative method of measuring SPL at the eardrum with the probe microphone (without using RECD). Specifically, the LoPresti reference suggests placing the probe microphone at a fixed distance from the sound source (5 mm). The expected shape of the SPL in the ear canal should have a notch at a frequency of a ¼ wavelength resonance of the ear canal. The LoPresti reference further proposes to locate the notch frequency F; calculate the quality factor Q of the notch; calculate the correction based on the Q and F values, and use the correction to compensate for the effect of the ear canal resonance. In practical use, the method proposed will not work, because the LoPresti reference addresses errors of SPL measured in the ear canal. The shape of the frequency responses of SPL in the ear canal will depend on many factors besides the ear canal resonances, so it could be very difficult, if not impossible, to identify the notch at an SPL response that is related to the ear canal resonance. FIG. 3 illustrates the problem of identifying the frequency F and the quality factor Q of the notch, caused by the ear canal resonance. The expected notch frequency F is between 6000 and 7000 Hz and can not be easily identified because the SPL responses depend on many factors other than the ear canal resonance.